To deduce mechanisms for electron transfer reactions of metalloproteins such as the non-heme iron proteins and the cytochromes, detailed knowledge of the nature of the electron transfer process is necessary. There exists a need for experimental verification of predicted trends in electron transfer rates as a function of the distance between oxidant and reductant centers. The predictions for such trends due to the mechanisms of adiabatic electron transfer or of tunneling are radically different, but experiments to date have not resulted in a determination of the limiting distance for adiabatic electron transfer. We intend to use well-defined, substitutionally inert transition metal complexes to investigate in a systematic manner the variation of electron transfer rates as the oxidant-reductant separation distance is increased. To fulfill the criterion of well-defined complexes with progressively larger radii, we will synthesize complexes of substitutionally inert Ru (II), Ru (III), and Co (III) with saturated ligands. These complexes will be rigid bimetallic steroid derivatives bound to a solid support. The variation in center-to-center distances will be approximately 9 to 14 A. Kinetics will be studied by conventional spectrophotometry. The results will be compared with predictions from theories of electron transfer, including that of Marcus for adiabatic outer-sphere electron transfer and theories of vibronically coupled electron tunneling.